A general problem during petroleum processing is separating desirable fractions of a petroleum (hydrocarbon) stream from other fractions that are less desirable or are desirable for a different purpose. A common example of a separation is to separate a lower boiling fraction, such as a diesel boiling range fraction, from a higher boiling fraction, such as a lubricant boiling range fraction. While separations based on boiling point are well understood, many desirable qualities in a petroleum fraction are not directly correlated with boiling point.
Liquid thermal diffusion provides a method for performing a liquid separation that is an alternative to boiling point based separations. U.S. Pat. Nos. 2,541,069 and 3,180,823 are early examples of using liquid thermal diffusion to separate hydrocarbon fractions, such as lubricant boiling range fractions. U.S. Pat. No. 3,180,823 also describes use of an additive to facilitate a liquid thermal diffusion process, and the withdrawal of multiple different fractions during a separation.
U.S. Pat. No. 6,783,661 describes a method of using liquid thermal diffusion for separation of a residue or bottoms fraction from a process for converting a distillate boiling range feed. The liquid thermal diffusion is used to separate the bottoms fraction based on viscosity index. A portion of the bottoms fraction can then be recycled for further processing.